This invention relates to an output circuit for portable generators, and more particularly to an output circuit for compact portable generators which supply high direct current output power.
Portable generators which are designed to supply high output power are generally required to have an output winding formed of coils large in diameter, and full consideration should be given such that an insulator which insulates the coils of the output winding are formed of a material which can withstand high voltage and large current.
In a portable generator which meets these requirements, such as one generating 30-40 A of direct current, an output winding having a large coil diameter has to be wound on a small stator core, which requires the use of a coil winding machine which is capable of winding with a large tensile force. Besides, large capacity type and large sized semiconductor components have to be used to control the output voltage from the generator, which makes the control circuit large in size and complicated in structure.
On the other hand, a generator for welding purposes as proposed by Japanese Provisional Utility Model Publication (Kokai) No. 62-142473 comprises two sets of output windings which are disposed to have their respective outputs combined together, only one output of which is controlled by means of a chopper such that welding current, i.e. the combined output can vary from a small value to a large value. This proposed generator can avoid an increase in the coil diameter of the output windings, and also enables designing the control circuit compact in size. However, the proposed generator is not capable of maintaining the output voltage at a constant level as required of a general purpose generator which supplies direct current.
In portable generators in general, the output circuit is provided with an overcurrent detector formed e.g. of a shunt resistance, in order to detect overcurrent due to overload on the circuit, which detector is connected in series with a load device such that when overcurrent is detected by the overcurrent detector, the supply of output current to the load device is cut off. However, the overcurrent detector is always supplied with part of output current supplied to the load device, there is power loss. Particularly in a generator which is required to supply high current output power such power loss is large and not negligible.
On the other hand, a device is known e.g. from Japanese Provisional Patent Publications (Kokai) Nos. 58-43016 and 64-83156, in which an FET (field effect transistor) as a switching element is connected in series to a load, and a voltage drop across the source and drain of the FET during conduction, which is caused by resistance between the source and the drain, is detected as a detected value of output current to thereby control the degree of conduction of the FET. If the technique of this proposed device is applied to the above proposed output circuit for portable generators, it should be possible to reduce the power loss.
Certainly, if the FET is employed in the output circuit of the portable generator, the supply of output current to the load can be cut off when excessive output current is detected to thereby protect the generator including the output circuit. However, if the overcurrent detection is repeated in such a manner that while the engine which drives the generator continues operating, the load device connected to the output circuit of the generator is disconnected therefrom to release the current supply cut-off state, and then the load device is again connected to the output circuit to bring the latter into an overloaded state and hence again effect cutting-off of the current supply, such repeated disconnection and connection can impair the generator and the load device.
Depending upon the structure of the load device, there is a possibility that the cause of overcurrent due to the load vanishes when the supply of output current from the generator is cut off upon detection of overcurrent (for example, a case where overcurrent is caused by part of the load device becoming hot) so that shortly after output current is again supplied from the generator, overcurrent again takes place. Therefore, the possibility is rather high that the occurrence of overcurrent due to such cause cannot be easily recognized by the driver of a vehicle in which the engine and the generator are installed, and the generator and the load device are impaired before the overcurrent is recognized by the driver.
Generally, the above output current cut-off is not effected immediately upon detection of an overcurrent state (overloaded state), but it is effected after such state has lasted over a short time period. This increases the possibility of damage to the generator and the load device.
Furthermore, if a smoothing capacitance is connected to the input side of the FET used in the output circuit for cutting off the output current supply upon occurrence of overcurrent, the smoothing capacitor is suddenly discharged to apply large current to the FET to impair same in the event that there occurs a short circuit across the load device or a battery to be charged by the generator, which is also a load device, is connected in reverse polarity to the output circuit.